430 J. GRAY 



{b) development, which usually depends on the exposure of the 

 egg to hypertonic sea-water. Let us now consider the second 

 or developmental phase of fertilization. 



In the normally fertilized egg, the visible change which 

 attends the second phase of fertilization, consists in the inclu- 

 sion into the egg-substance of the head and middle piece of the 

 spermatozoon ; soon after this an aster appears in the vicinity 

 of the sperm-nucleus. Eventually the male and female pro- 

 nuclei, having approached each other, fuse together, and cell- 

 division begins. We must suppose that as soon as the cell 

 wall of the spermatozoon and that of the egg at the point of 

 attachment are broken down ^ then the body of the spermato- 

 zoon will be very rapidly drawn into the egg by surface tension. 

 On the mechanism of this process no experimental facts are 

 available. 



Once the sperm has been drawn into the egg, we can continue 

 our experimental analysis. The experiment of Kupeh\aeser (19) 

 shows that the sperm-nucleus plays no essential role, since 

 although in certain cases it rapidly degenerates yet normal 

 segmentation occurs : again in artificial parthenogenesis com- 

 plete development takes place without any male pro-nucleus. 

 Now, the only other visible structure which is associated with 

 the male nucleus is the male aster. In Lillie's experiment with 

 the centrifuged eggs of Nereis, it was found that eggs from 

 which the sperm had been removed failed to develop a bipolar 

 mitotic spindle ; only one aster — the female aster — was 

 present ; otherwise the nuclear behaviour of the egg was normal. 

 Again, in Kupelwieser's experiment, although the male nucleus 

 degenerated the male aster remained functional. From a study 

 of normal fertilization we therefore suspect that the develop- 

 mental phase of fertilization is associated vdth. the existence 

 of two asters, one belonging to the female nucleus, and one 

 introduced into the egg by the spermatozoon — or which comes 

 into being when the male nucleus enters the egg. 



» Until this occurs the two cells will remain essential^ distinct from 

 each other. Mere agglutination m a common matrix would not produce 

 actual incorporation of the sperm and the egg. 



